The dramatic and ever-increasing emergence of many relevant strains of bacteria resistant to traditional antibiotics is now a major issue in human health. Antibiotic resistance has arisen due to the extensive clinical use of classical antibiotics. Thus, at best, antibiotics are progressively demonstrating decreased efficacy; at worst, there has been an upsurge of untreatable infections, such as multi-resistant tuberculosis and vancomycin-resistant Enterococcus strains. Consequently, the cost of treating nosocomial infections through extended hospitalization and increasingly aggressive therapy has risen to an estimated $30 billion in the U.S. Therefore, there is an economic incentive to adopt novel antibiotics. In addition, the threat of bioterrorism, that is the ability to easily engineer new strains of bacteria with deadly consequences to humans, must be dealt with. Compared to existing antibiotics, antimicrobial peptides show great potential as a radically new structural class of antibiotics, with both novel modes of action as well as different cellular targets. The development of resistance to membrane active peptides whose sole target is the cytoplasmic membrane is not expected since this would require substantial changes in the lipid composition of cell membranes of microorganisms. The advantages of cationic antimicrobial peptides are their ability to kill target cells rapidly, their unusual broad spectrum activity and their activity against some of the more serious antibiotic-resistant pathogens isolated in clinics. The major barrier to the use of antimicrobial peptides as antibiotics has been their toxicity or ability to lyse eukaryotic cells. We have taken an approach of systematic alteration in the amphipathicity, hydrophobicity and structure of two different classes of antimicrobial peptides by single L- and D- amino acid substitutions. With this approach we were able to dissociate anti-eukaryotic activity from antimicrobial activity, i.e. increase the antimicrobial activity and dramatically reduce or eliminate toxicity to normal cells (as measured by hemolytic activity). We have discovered lead compounds in two different structural classes of antimicrobial peptides, a 14-residue cyclic 2-sheet peptide and a 26-residue 1-helical peptide with clinical potential as broad spectrum antibiotics. The simultaneous development of both classes of compounds will most rapidly advance our knowledge of the mechanism of action of these peptides and the common requirements for selectivity for microbial membranes. Further optimization of our de novo designed lead compounds is required to ensure we obtain the best antimicrobial activity while maintaining a high therapeutic index. Key goals: 1) to determine the best type of positively-charged residue in the center of the non-polar face to enhance specificity on the cyclic 2- sheet and 1-helical peptides; 2) having selected the best positively-charged residue to eliminate hemolytic activity, can we increase hydrophobicity to improve antimicrobial activity while maintaining a high therapeutic index; 3) to demonstrate that our peptides are non-toxic to a series of human cell lines; 4) to show by structural determination (solid-state NMR) the location of our two classes of antimicrobial peptides in the membrane; 5) to demonstrate in vivo efficacy of our peptides against Pseudomonas aeruginosa challenge in two animal models; 6) to extend our studies to animal models of other serious bacterial infections. [unreadable] [unreadable] PUBLIC HEALTH RELEVANCE: The dramatic and ever-increasing emergence of many relevant strains of bacteria resistant to traditional antibiotics is now a major issue in human health. Antibiotic resistance has arisen due to the extensive clinical use of classical antibiotics. Consequently, the cost of treating nosocomial infections through extended hospitalization and increasingly aggressive therapy has risen to an estimated $30 billion in the U.S. Thus, there is an economic incentive to adopt novel antibiotics. [unreadable] [unreadable] [unreadable]